In colonies of social Hymenoptera (which include all ants, as well as some wasp andbee species), only queens reproduce whereas workers generally perform other tasks.The evolution of worker's reproductive altruism can be explained by kin selection,which states that workers can indirectly transmit copies of their genes by helping thereproduction of relatives. The relatedness between queens and workers may howeverbe low, particularly when there are multiple queens per colony, which limits thetransmission of copies of workers genes and increases potential conflicts betweencolony members. In this thesis, we investigated the link between social structurevariations and conflicts, and explored the mechanisms involved in variation of colonyqueen number in ants.According to kin selection, workers should rear the brood they are most related to.In social Hymenoptera, males are haploid whereas females (workers and queens) arediploid. As a result, workers can be up to three times more related to females thanmales in some colonies, where they should consequently favour the production offemales. In contrast, queens are equally related to daughters and sons in all types ofcolonies and therefore should favour a balanced sex ratio. In a meta-analysis acrossall studies of social Hymenoptera, we showed that colony sex ratio is generallylargely influenced by workers. Hence, the evolution of social structures where queensand workers are equally related to males and females may contribute to decrease theconflict between the two castes over colony sex ratio.Another conflict between queens and workers can occur over male production.Many species contain workers that still have the ability to lay haploid eggs. In somesocial structures, workers are on average more related to sons of queens than to sonsof other workers. As a result, workers should eliminate worker-laid eggs to favourqueen-laid eggs. We showed that in the ant Formica selysi, workers eliminate moreworker-laid than queen-laid eggs, independently of colony social structure. Theseresults therefore suggest that worker policing can evolve independently fromrelatedness, potentially because of costs of worker reproduction at the colony-level.Colony queen number is a key parameter that influences relatedness betweengroup members. Queen body size is generally linked to the success of independentcolony foundation by single queens and may influence the number of queens in thenew colony. In the ant F. selysi, single-queen colonies produce larger queens thanmultiple-queen colonies. We showed that this association results from genes ormaternal effects transmitted to the eggs. However, we also found that queensproduced in colonies of the two social forms did not differ in their general ability tofound new colonies independently. Queen body size may also influence queendispersal ability and constrain small queens to be re-adopted in their original nestafter mating at proximity. We tested the acceptance of new queens in another antspecies, Formica paralugubris, which has numerous queens per colony. Our resultsshow that workers do not discriminate between nestmate and foreign queens, andmore generally accept new queens at a limited rate.To conclude, this thesis shows that mechanisms influencing variation in colonyqueen number and the influence of these changes on conflict resolution are complex.Data gathered in this thesis therefore constitute a solid background for furtherresearch on the evolution and the maintenance of complex organisations in insectsocieties.